This present invention is directed to electrochemical cells that incorporate zinc in the anode. The invention is particularly directed to alkaline electrochemical cells wherein zinc powder is the electrochemically active material in the anode.
Electrochemical cells, having either a prismatic or cylindrical shape, are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. The cylindrical batteries used in these devices typically employ a tubularly shaped metal container to house two electrodes, a separator, a quantity of electrolyte and a closure assembly that includes a current collector. Typical electrode materials include manganese dioxide as the electrochemically active material in the cathode and zinc as the electrochemically active material in the anode. An aqueous solution of potassium hydroxide is a common electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and anode.
Commercially available cylindrical alkaline batteries use an anode that includes zinc powder. The anode includes a gel, zinc powder and an aqueous electrolyte. The gel is formed by contacting a powdered gelling agent with a liquid, such as the cell's aqueous based electrolyte, which is readily absorbed by the gelling agent to form the gel. The zinc particles are mixed with and uniformly dispersed throughout the gel so that particle-to-particle contact establishes an electrically conductive matrix throughout the anode. Unfortunately, the gel negatively impacts the cost and service performance of the cell in at least four ways. First, the gel occupies space that could be used to hold additional zinc powder that would increase the electrochemical capacity of the anode. Second, the space occupied by the gel could be dedicated to holding reaction products that accumulate within the anode during discharge thereby delaying or avoiding a detrimental phenomenon known as “anode shut down”. The anode shuts down when the anode's reaction products form a semi permeable layer on the surface of the anode. The layer increases the cell's polarization thereby reducing the time that the battery can power a device. Third, the gelling agent causes the anode to be difficult to process in high speed assembly machines. In particular, the gel tends to form lumps within the piping that is used to convey the anode from the anode manufacturing area to the cell assembly area. The lumps disrupt the anode distribution process which ultimately increases the cost of manufacturing the cells. Fourth, the gelling agent slows down or prevents the transmission of hydrogen gas through the anode thereby inhibiting the cell's ability to safely release hydrogen from the cell. If the gas, which is generated in the anode, cannot escape from the anode and then the cell, the cell's internal pressure may increase and activate the cell's safety vent which effectively terminates the useful life of the cell.
As disclosed in the three patents described below, battery manufacturers have sought ways to eliminate the gel from the anode of alkaline cells. U.S. Pat. No. 6,150,052, discloses an alkaline cell design that utilizes a plurality of stacked zinc discs instead of particulate zinc powder. The spacing between the discs provides room for reaction products to accumulate during discharge of the cell thereby delaying or avoiding the creation of a reaction product skin that could cause an increase in the cell's polarization and lead to premature cell failure. While the use of zinc discs is an effective way to avoid an increase in the anode's polarization when compared to similar cells that utilize particulate zinc powder, the discs must be punched from sheets of zinc, oriented and stacked before inserting them into the cell's separator lined cavity. These additional steps increase the cost of manufacturing the cell and thus the “stacked disc” cell construction described above may not be an economically viable option for battery manufacturers that produce millions of batteries each year on high speed assembly machines. Another example of a cell design that improves the battery's run time by delaying or avoiding the formation of a reaction product skin on the anode is disclosed in U.S. Pat. No. 6,627,349. One of the embodiments in this patent utilizes a slotted zinc tube that has a spine and a plurality of ribs supported by the spine. In another embodiment, the anode's electrochemically active material is a coiled strip of zinc having overlapping layers. Unfortunately, the embodiments shown in U.S. Pat. No. 6,627,349 require process steps to cut slots in the zinc tube or cut and coil the strip of zinc. These steps increase the cost of manufacturing the cell relative to cells that utilize particulate zinc powder. U.S. Pat. No. 6,673,494 discloses an alkaline cell that utilizes an expanded zinc mesh anode in place of an anode containing zinc powder and a gelling agent. The specification teaches that a gelling agent is not desirable because it may interfere with ion transport. More specifically, the specification suggests eliminating or decreasing the amount of the gelling agent in order to simplify the battery design and reduce costs. The inventors teach replacing the mixture of zinc powder and gelling agent with an expanded zinc mesh which may be folded, layered or coiled. Although the use of a zinc mesh is described as offering advantages over the use of a conventional anode formed from a mixture of zinc powder, gelling agent and electrolyte, the zinc mesh cannot be conveyed and dispensed in the machines currently used by the major battery manufacturers to assemble alkaline cells at rates that exceed 300 cells per minute. Converting from an anode that uses zinc powder to one that uses expanded zinc mesh would require the battery manufacturers to invest substantial sums of money in the design, fabrication and installation of new anode dispensing equipment. Consequently, the use of expanded zinc mesh may not be economically viable for established producers of cylindrical alkaline batteries.
Therefore, there exists a need for an alkaline cell that incorporates a gel free anode having zinc powder as the electrochemically active material.